Update: I made a few minor tweaks to this guide to (hopefully) avoid some confusion. This guide works for KDE SC 4.6 (and should for any future KDE developments hosted in the repo it uses.) I tried this on a fresh install (and completely dist-upgraded.)

This is an unofficial, unsupported how-to for those who wish to try KDE SC 4.6.x on their systems. Do not try this unless you know what you are doing!
This how-to uses a third-party repo found at http://qt-kde.debian.net

Step 1
Add these lines to a .list file in /etc/apt/sources.list.d/
(e.g. /etc/apt/sources.list.d/kde.list)